Sensitivity of single units in the LSO of decerebrate cat to sinusoidally amplitude modulated tones

Abstract

Fluctuations in amplitude are a common component of behaviorally important sound stimuli. Amplitude modulation (AM) is encoded by the peripheral auditory system in the timing of discharge spikes, and, more centrally, in the discharge rate. The mechanism producing this transformation from a time- to rate-based code is not known, but recent modeling efforts have suggested a role for neurons with response characteristics consistent with cells in the lateral superior olive (LSO). The responses of single units in the LSO of unanesthetized decerebrate cat were recorded to monaural sinusoidally amplitude modulated (SAM) tones by systematically varying sound level and modulation frequency (fm), and are described in terms of synchronization to the envelope and average discharge rate as a function of fm. LSO units typically synchronize strongly to low fm, and discharge preferentially (i.e. more strongly) over a small range of fm in response to low level SAM tones. At higher sound levels synchronization decreases and response rate increases until most or all modulation in the response is lost. These results contrast with responses recorded in the barbiturate-anesthetized cat, which tend to respond to most low-frequency modulations, and are consistent with LSO as an intermediate processing stage between the peripheral temporal- and central rate-based code for AM sounds.